Die casting is a technology in which a melt of metallic material (non-ferrous metal such as zinc, aluminum or magnesium or an alloy thereof) is injected under elevated pressure into a die or mold mounted in a die casting machine to allow the metallic material to solidify therein, the solidified metallic material being thereafter taken out of the die. Die casting is excellently characterized in that it is productive and products diecast are high in dimensional accuracy, superior in strength and aesthetically fine in appearance, requiring mechanical machining the least.
Semi-solid die casting techniques have further been developed in which instead of injecting a melt of metallic material into a die, a semi-solid material (semi-solid slurry) formed and prepared separately is received in a sleeve disposed in front of the die and the semi-solid slurry is injected into the die by a plunger.
To wit, as recent techniques for yielding quality cast products, semi-solid die casting techniques (thixocasting or semi-molten casting and rheocasting or semi-solidified casting) are accepting attention.
Rheocasting is a method of cooling an alloy from its liquid state while it is being agitated to grow primary crystal in the form of particles, and molding when a certain rate of solidification is reached, and is also called semi-solidified casting. As for thixocasting which is also called semi-molten casting, an alloy molten is once solidified while it is being agitated to form a billet which when cast is heated again to form a body in solid and liquid coexisting state, the body being then molded. Thixocasting has not only a problem that such billets are expensive which are to be controlled of structure and are special but also has those in respect of energy saving and recycling in that billets are to be re-molten into a semi-molten slurry to be cast. One that is once cast cannot be re-used upon re-melting. Presently, rheo-casting is mainstream.
As one of rheocasting processes which are in a stage of being developed into commercial use, there is a NRC (New Rheo-Casting) process developed by Ube Industries, Ltd with proven results of an aluminum alloy for cast iron in volume production of undercarriage parts and brake calipers of automobiles.
The NCR process is a process in which a low-temperature melt while it is not agitated is poured with a slurry cap and after crystallization of a given amount of solid phase, the slurry in solid and liquid coexisting state is put into an injection sleeve for injection filling.
The NRC process, however, requires time in forming the semi-solidified slurry, necessitates large and costly equipment and has a limit in micronizing a spherical crystal due to an insufficient number of occurrences of nucleation.
A present inventor has separately developed cup processes such as a process (nano-casting process) in which to form a slurry inexpensively, quickly and simply and to increase the number of occurrences of nucleation, agitation is produced electromagnetically (Patent Document 1), and a self-agitating method (Patent Document 2).
The cup processes are a semi-solid die casting method in which a melt of metallic material is poured into a cup to form a semi-solidified slurry therein, and the semi-solidified slurry is moved into a sleeve, the semi-solidified slurry being thereafter injected or molded into a die.
On the other hand, there is a process in which a melt is poured into a sleeve and semi-solidified into a slurry therein, the semi-solidified slurry in the sleeve being thereafter pushed into a die. This process is called a sleeve process (e.g. Patent Document 3).
The conventional die casting techniques including semi-solid die casting processes, however, has a limitation in thickness, i.e. a limitation in thickness of products that can be made. The limitation in thickness has been described as t=0.6 to 1.0 mm (e.g. http://www.nagas.co.jp/technology/index.html).
Indeed, no diecast product exists having a thickness of less than 0.6 mm.
It can be noted here that Patent Document 4 proposing a technique of making a separator by a semi-solid die casting process, describes in claim 6 that “separator has a plate thickness of 0.4 mm or less in its thinnest area.”
The plate thickness mentioned there, however, is that which as described (in paragraph 0032 of Patent Document 4) as “Since the separator is formed with grooves on their both sides, it is the thinnest in an area where the groove on the one side and the groove on the other side are crossed”, is in fact a distance in an area where the groove on the one side and the groove on the other side are crossed, and further which as described (in paragraph 0053) as “A flat plate may be molded and be thereafter formed with the grooves by mechanical machining. Further, the flat plate after it is molded is formed with the grooves by stamping”, is in fact of a thickness of an area where the grooves are created by machining after die casting and not of a thickness as cast.
In an attempt to micronize a primary crystal α, a technique is proposed in Patent Document 5.
It is a technique in which a melt of metallic material is poured along an inclined plate into a container and a process in which cooling is controlled by the inclined plate cooled for nucleation to form a semi-solid slurry. Attempting, however, to optimize the temperature to which the inclined plate is cooled, the distance of its contact with the melt, the angle of its inclination and the temperature at which the melt is poured, fails to stabilize granulation and to render the process operational. There is the restriction that raising the rate of cooling does not allow increasing the rate of flow of the melt along the cooled inclined plate. The melt if reduced in rate of flow comes to flow while meandering. Because of the difficulty to hold a steady state, it is the present situation that not only does the resulting grain or particle size of primary crystal α vary widely in a range between 100 and 200 μm but also low rate of flow of the melt causes oxidation of its surfaces, leaving the process commercially nonoperational.